An energy storage device, especially a lithium secondary battery, has been widely used recently for a power source of an electronic device, such as a mobile telephone, a notebook personal computer, etc., and a power source for an electric vehicle and an electric power storage. There is a high possibility that the battery mounted on such an electronic device or vehicle is used at midsummer high temperatures or under an environment warmed by generation of heat of the electronic device. In a thin electronic device, such as a tablet device, an ultrabook, etc., a laminate-type battery or a prismatic battery using a laminate film, such as a an aluminum laminate film, etc., for an outer packaging member is frequently used; however, since such a battery is thin, a problem that the battery is easily deformed due to expansion of the outer packaging member or the like is easily caused, and the matter that the deformation very likely influences the electronic device is problematic.
A lithium secondary battery is mainly constituted of a positive electrode and a negative electrode, each containing a material capable of absorbing and releasing lithium, and a nonaqueous electrolytic solution containing a lithium salt and a nonaqueous solvent; and a carbonate, such as ethylene carbonate (EC), propylene carbonate (PC), etc., is used as the nonaqueous solvent.
A lithium metal, a metal compound capable of absorbing and releasing lithium (e.g., a metal elemental substance, a metal oxide, an alloy with lithium, etc.), and a carbon material are known as the negative electrode of the lithium secondary battery. In particular, a nonaqueous electrolytic solution secondary battery using, as the carbon material, a carbon material capable of absorbing and releasing lithium, for example, coke or graphite (e.g, artificial graphite or natural graphite), etc., is widely put into practical use. Since the aforementioned negative electrode material stores/releases lithium and an electron at an extremely electronegative potential equal to the lithium metal, it has a possibility that a lot of solvents are subjected to reductive decomposition especially at high temperatures, and a part of the solvent in the electrolytic solution is reductively decomposed on the negative electrode regardless of the kind of the negative electrode material, so that there were involved such problems that the movement of a lithium ion is disturbed due to deposition of decomposition products, generation of a gas, or expansion of the electrode, thereby worsening battery characteristics, such as cycle property, especially at high temperatures, etc.; and that the battery is deformed due to expansion of the electrode. Furthermore, it is known that a lithium secondary battery using a lithium metal or an alloy thereof, or a metal elemental substance, such as tin, silicon, etc., or a metal oxide thereof as the negative electrode material may have a high initial battery capacity, but the battery capacity and the battery performance thereof, such as the cycle property, may be largely worsened especially at high temperatures since the micronized powdering of the material may be promoted during cycles, which brings about accelerated reductive decomposition of the nonaqueous solvent, as compared with the negative electrode formed of a carbon material, and the battery may be deformed due to expansion of the electrode.
Meanwhile, since a material capable of absorbing and releasing lithium, which is used as a positive electrode material, such as LiCoO2, LiMn2O4, LiNiO2, LiFePO4, etc., stores and releases lithium and an electron at an electropositive voltage of 3.5 V or more on the lithium basis, it has a possibility that a lot of solvents are subjected to oxidative decomposition especially at high temperatures, and a part of the solvent in the electrolytic solution is oxidatively decomposed on the positive electrode regardless of the kind of the positive electrode material, so that there were involved such problems that the movement of a lithium ion is disturbed due to deposition of decomposition products or generation of a gas, thereby worsening battery characteristics, such as cycle property, etc.
Irrespective of the situation, the multifunctionality of electronic devices on which lithium secondary batteries are mounted is more and more advanced, and power consumption tends to increase. The capacity of lithium secondary battery is thus being much increased, and the space volume for the nonaqueous electrolytic solution in the battery is decreased by increasing the density of the electrode, or reducing the useless space volume in the battery, or the like. In consequence, it is a situation that the battery performance at high temperatures is easily worsened due to even a bit of decomposition of the nonaqueous electrolytic solution.
PTL 1 proposes a secondary battery containing an electrolytic solution including a diisocyanate compound as a nonaqueous electrolyte secondary battery capable of exhibiting a swelling suppression effect at the time of high-temperature storage.
PTL 1: JP-A 2011-14379